Single-cell lithium-ion batteries as a power source are widely used in mobile phones. Lithium-ion batteries are based on chemical energy storage, and the energy density is very large. Therefore, there are inherent safety risks associated with bare lithium batteries, leading to the need for battery protection circuits. A lithium battery protection IC (integrated circuit) needs to have high precision measurement of the voltage value to achieve effective over-voltage protection, under-voltage protection, charge overcurrent protection, short circuit protection, and discharge overcurrent protection, etc.
Embedded cell phone batteries, also known as integrated batteries, have become a prevalent design feature in new phone models today, especially in smartphone models. Proponents of embedded batteries say they allow for smaller, sleeker, more ergonomic phone designs, as well as extended battery life. However, embedded batteries prevent the customers from easily replacing the battery. Embedded batteries also place more demand on the battery protection circuit. For example, these embedded batteries require more accurate and efficient charge and discharge protection. In addition, the battery protection circuit for embedded batteries also needs to provide forced battery shutdown function. For example, when the mobile phone undergoes testing in the factory, the battery protection circuit needs to shut down the embedded battery to allow testing using an external power supply. After the testing is completed, the battery protection circuit reconnects the embedded batter to power the mobile phone.
FIG. 1 is a schematic diagram illustrating a cell phone system 100 including a conventional battery protection circuit. As shown in FIG. 1, cell phone system 100 includes a cell phone circuit block 110, a battery protection circuit 120, and NMOS transistors NMOS1 and NMOS2. Cell phone circuit 110 is connected to battery protection circuit 120 and two NMOS transistors NMOS1 and NMOS2. These components are often disposed on a cell phone mother board. System 100 also includes an embedded battery 130 and an external power V1. The external power can come from a test system for functional testing of the cell phone, or from a battery charger for providing power for battery charging. In FIG. 1, battery protection circuit 120 includes terminals VDD, CTL, VM, CO, and DO. VDD is a power input terminal of the battery protection circuit, which is connected to the positive terminal B+ of the battery. In battery protection circuit 120, terminal DO is for providing a discharge control signal to NMOS1, and terminal CO is for providing a charge control signal to NMOS2. Under normal operating conditions, control signals at DO and CO are high, and both NMOS1 and NMOS2 are turned on. As a result, cell phone circuit 110 is connected to embedded battery 130, and the battery can be both charged and discharged. In FIG. 1, P− is a negative terminal of external power V1, and P+ is the positive terminal of the external power. The phone load is connected between P+ and P−. A resistor R1 is connected between terminal VM and P−, and provides current limiting protection. The CTL terminal receives a signal from cell phone circuit 110, e.g., from an input/output (I/O) port of the mobile phone control circuit. The signal at the CTL terminal can vary between a logic high level, e.g., 1.8V and a logic low of the voltage at P−. When the signal at CTL is high, both DO and CO are low; so NMOS1 and NMOS2 are turned off, disconnecting the battery from the phone. At this time, the mobile phone can receive power from the external power source V1 and undergo functional testing. After the test is completed, the CTL is set low, and the DO and CO terminals are restored to high. At this time, NMOS1 and NMOS2 are turned on, and the battery is reconnected to the mobile phone to provide operating power.
The inventors have observed a flaw in the protection method in system 100. Generally, battery protection circuit 120 uses the B− terminal of the battery as a ground reference. On the other hand, the signal provided to the CTL terminal by cell phone circuit 110 uses the P− terminal as a ground reference. During testing, NMOS1 and NMOS2 are turned off, and the two ground terminals B− and P− are isolated from each other. The P− terminal is connected to the external power source, but the B− terminal may be electrically floating relative to P−. When the cell phone circuit attempts to turn on NMOS1 and NMOS2 by setting the signal at CTL to low relative to terminal P−, the protection circuit may not be able to recognize the CTL signal being low, and may fail to turn on NMOS1 and NMOS2 to restore the normal operation.
FIG. 2 is a schematic diagram illustrating a mobile device system 200 including another conventional battery protection circuit. As shown, system 200 is similar to system 100 in FIG. 1, including a cell phone circuit 210, a battery protection circuit 220, an embedded battery 230, and an external power source V1. However, the charging and discharging protection NMOS transistors in FIG. 1 are replaced by PMOS transistors in system 200 in FIG. 2. PMOS transistors PMOS1 and PMOS2 are connected in series between the positive terminal B+ of the battery and the positive terminal P+ of the cell phone circuit. The negative terminal B− of the embedded battery and the negative terminal P− of the external power are directly shorted, maintaining a common ground. Because of the common ground, when the signal at CTL is high, the battery protection circuit can accurately identify this high state and set the signals at DO to CO to high, which turns off PMOS1 and PMOS2 and disconnects the battery and the phone. Under this condition, the phone can directly receive power from the external power supply V1 during functional testing. When external power V1 is removed and the signal at CTL is low relative to P−, it is also low relative to B−, because of the common ground. The battery protection circuit sets DO and CO to low and turns on PMOS1 and PMOS2. Under this condition, the mobile phone is powered by the embedded battery for normal operation.
The battery protection method illustrated in FIG. 2 appears to resolve the floating ground problems depicted in FIG. 1. However, the inventors have identified limitations associated with method illustrated in FIG. 2. PMOS1 and PMOS2 are also used for the battery to provide power to the cell phone and for charging the battery from a charger. To satisfy the increasing demand for fast charging operation in a cell phone, the on-resistances Rds from the charging and discharging protection transistors must be kept low, e.g., 15 mΩ or low. Because of the inherent low mobility of PMOS transistors, substantially larger PMOS transistors must be used to implement the solution in FIG. 2 to obtain low on-resistances compared with FIG. 1, which uses NMOS transistors. For example, the battery protection circuit is usually implemented as an integrated circuit and the protection transistors are usually implemented as discrete high-voltage transistors. Because of their large size, the PMOS transistors are not suited for thin mobile phones which require small dimension IC packages. The larger PMOS transistors are also more costly than the NMOS transistors.
Therefore, there is a need for improved battery protection methods for mobile devices in small packages.